At least 20 people have died in an accident on a Russian nuclear submarine when a fire extinguishing system was activated by mistake.

Russian Pacific Fleet spokesman Igor Dygalo said both sailors and shipyard workers died in the accident, which occurred during sea trials.

The dead were suffocated by freon gas, which is used to put out fires as it removes oxygen from the air.

The vessel was not damaged and there was no radiation leak, Mr Dygalo said.

It is the worst incident for the navy since the sinking of the Kursk submarine in 2000, which left 118 dead.

Injured evacuated

Military prosecutors are investigating the incident, but the name and class of the submarine involved have not been officially revealed.

There were 208 people on board at the time, 81 of whom were servicemen. The dead are reported to be six sailors and 14 civilians.

Graphic from Russian televisionRussian TV speculated on how the accident might have happened

Twenty-one injured people, reported to be suffering from various degrees of poisoning, were transferred from the submarine to the destroyer Admiral Tributs and taken to hospitals near Vladivostok.

The dead were transferred to morgues nearby after the vessel, which had been undergoing sea trials in the Sea of Japan, returned to port at Bolshoi Kamen, near Vladivostok, in the far eastern Primorsky territory.

Capt Dygalo said the incident happened after the fire extinguishing system "went off unsanctioned".

The BBC's Richard Galpin in Moscow says that in normal cases of a fire on board a submarine, the area would be evacuated and sealed off before pumping in freon gas.

The gas removes oxygen from the air - to put out the fire - but if anyone is still trapped inside that area, they face suffocation.

Vladimir Markin, an official from Russia's top investigative agency, later said forensic tests had confirmed that freon was the cause of death.

Reports say the incident occurred in the nose of the submarine. The nuclear reactor, which is in the stern, was not affected.

"I declare with full responsibility that the reactor compartment on the nuclear-powered submarine is working normally and the radiation background is normal," Capt Dygalo said, quoted by Itar-Tass news agency.

Indian link

A shipyard source told the RIA Novosti news agency the vessel was the K-152 Nerpa, an Akula-class submarine, but this has not been confirmed.

Twenty die on Russian submarine

The Nerpa is due to be leased to the Indian navy, and Indian naval personnel were due to travel to Vladivostok earlier this month to train on board the submarine ahead of its transfer, according to the website Indian Defence.

Russian President Dmitry Medvedev is being kept fully informed about the incident, his press service said.

Deputy Defence Minister Alexander Kolmakov and Navy Commander-in-Chief Vladimir Vysotsky are flying to the scene of the incident.

Russia's worst submarine disaster happened in August 2000, when the nuclear-powered Kursk sank in the Barents Sea. All 118 people on board died.

The then president, Vladimir Putin, was criticised for being slow to react to the incident and reluctant to call in foreign assistance.